from the Equatorial Surface of a Magnet. 169 



cidences in the first experiment. It appears thus that in the 

 third experiment, as in the second, the optical effects of mag- 

 netization fall under two cases : — 



(1) Between grazing and principal incidences the right- 

 handed current conspires with a left-handed rotation of the 

 first Nicol, and so forward consistently. 



(2) Between principal and normal incidences, the preceding 

 law is simply reversed ; the right-handed current conspires 

 with a right-handed rotation of the first Nicol, and so forward. 



11. Fourth Experiment. — The two Nicols are placed initially 

 at pure extinction, the plane of polarization of the light in- 

 cident on the mirror being perpendicular to the plane of inci- 

 dence. All the other arrangements and the procedure are 

 as in the third experiment : the second Nicol remains fixed ; 

 and the first Nicol is displaced through a very small angle 

 from the position of extinction, first righthandedly and then 

 lefthandedly. The results are very similar to those obtained 

 in the first experiment. 



(1) The light restored by a right-handed rotation of the 

 first Nicol is always weakened by a right-handed current, and 

 always strengthened by a left-handed current. 



(2) The light restored by a left-handed rotation of the first 

 Nicol is always strengthened by a right-handed current, and 

 always weakened by a left-handed current. 



Very near grazing incidence, between 90° and 85°, the 

 effects are insensible ; about incidence 85° they are very faint, 

 but regular and quite distinct ; they increase in strength quite 

 evidently through the incidences 80°, 75°, 70°, to somewhere 

 between 65° and 60°, where they are, I think, as clear and as 

 strong as those obtained in the first experiment ; they then 

 diminish gradually to incidence 30°, where they are very 

 faint, but still somewhat stronger than at 85°. 



It appears thus that, in the fourth experiment, the right- 

 handed current conspires always with a left-handed rotation of 

 the first Nicol. 



12. The four experiments which have been described were 

 repeated at several incidences with mirrors of steel. Some 

 finely polished knife-blades were tried, and several masses of 

 other forms. The best was a small bar-magnet of hard steel, 

 which had one of its narrow faces polished on a cutler's wheel 

 (one of those large wheels used for sword-blades). The curva- 

 ture of this mirror was inconsiderable ; and its polish was ex- 

 tremely fine. The arrangements were as in the diagram (2), 

 the bar being laid stably from pole to pole of the horseshoe. 



All the old effects were recovered regularly. Only one 

 thing new attracted my notice, which was, the greater superi- 



